Push-to-talk (PTT) communication systems utilize a single radio channel to communicate in both directions, switching between sending and receiving by operating a button on the terminal (typically a handset). The classic example of a push-to-talk communication system is a group of walkie-talkies, which allows alternating communication between two or more users which are in each other's radio range.
Peer-to-peer (PTP) is a communication model in which each party has the same capabilities and each party can initiate a communication session. A pair of walkie-talkies also constitutes a peer-to-peer system.
A well-known communication system suitable for push-to-talk operation is TETRA (TErrestrial Trunked RAdio), which is used in many countries by police forces, emergency services, the national army, and/or train companies. TETRA-based systems for public order and safety are usually known by a national name, such as C2000 in the Netherlands.
TETRA allows one-to-one and one-to-many connections. Although TETRA may use a cellular network, it can also operate in Direct MOde (DMO) in which no cellular network is used and direct (i.e. peer-to-peer) push-to-talk communication between the handsets takes place. In either mode, a so-called dispatcher may operate a handset so as to relay information and coordinate the operations of, for example, an emergency team.
Unfortunately TETRA and similar communication systems have the disadvantage that the wireless connections between the terminals (that is, the handsets) are not always good, resulting in signal distortions and an insufficient audio quality. This is particularly the case in buildings, tunnels and mines, where walls block or at least severely attenuate the signal transmission. It will be clear that a certain minimum required audio quality is essential for communication between members of an emergency team, and that a lost wireless connection may have fatal consequences.
It is therefore highly desirable that a good signal quality of the wireless communication within a push-to-talk communication system is maintained, both in cellular operating conditions in which a cellular network is used, and in direct mode operating conditions, that is, without the assistance of a cellular network. It is known per se to use booster circuits for boosting the signal level of each individual terminal so as to improve the signal quality. However, this requires modifying each terminal, which is relatively expensive, and may still not result in a satisfactory minimum audio quality level. In addition, typical known power booster circuits are unsuitable for push-to-talk applications.
Korean Patent Application KR 2000 0067527, for example, discloses a power booster circuit for a portable CDMA (Code Division Multiple Access) terminal in a cellular communication system using cells and base stations. This known power booster circuit, which has a variable gain and uses several distinct frequencies, is not suitable for push-to-talk systems which use a single frequency for both transmission and reception. In addition, every portable terminal will have to be provided with this known power booster circuit for it to be effective.
European Patent Application EP 0 599 659 discloses a booster for use with a portable radio apparatus. The power amplifier of the booster is unidirectional, thus making the known booster unsuitable for push-to-talk systems. In addition, the power amplifier is switched on and off in response to a control signal generated by a control signal generator circuit of the portable radio apparatus. Accordingly, the known booster is not suitable for portable radio apparatus which is not fitted with such a control signal generator circuit, and can therefore not be used with existing portable radio apparatus.
Furthermore, International Patent Application WO 00/72606 discloses a system for communication within an energy-transmission-limited environment. RF transceivers are distributed throughout the site and are at each location connected to a control unit which provides both power and allows communication. This known arrangement requires a networked structure and a software-controlled architecture and is therefore relatively complicated. In addition, this known arrangement cannot be moved easily.
Cell enhancers are known that offer signal enhancement of cellular communication. However, such cell enhancers disturb the existing cell planning of the network and therefore require a careful tuning to minimise this disturbance. They are therefore not suitable for ad-hoc coverage enhancement. In addition, they are unsuitable for enhancing direct mode communication.